1. Field of the Invention
The present invention generally relates to a device in a vehicle damper that comprises a damping medium-filled damper body in which an element forming a seal against the damper body moves with a reciprocating stroke caused by relative movement between two parts of the vehicle. The element forming a seal against the damper body divides the damper body into two chambers and has a low-pressure side and a high-pressure side owing to the pressure differences in the damping medium under the induced movement. One or more valve and connecting arrangements couple the two damper chambers to a pressurizing element common to both of the chambers so that even in the low pressure chamber there is a positive pressure acting at all times, so that the damping force characteristic in both stroke directions can be adjusted separately and independently of one another. A continuous electrical signal, which is based on events registered by sensors and adjusted by a microprocessor control unit, controls a first part of the damping force characteristic of the damper by regulating flow through a first damping duct in the valve and connecting arrangement(s). A second part of the damping force characteristic of the damper is controlled by a further valve that is separated from the valve and connecting arrangement and that is located in a second damping duct.
2. Description of the Related Art
Electrically controlled valves in a damper are disclosed, for example, by SE 443622, SE 466514 and GB 2378231. The flow through these valves is determined by an electrically controlled pressure regulator, which forms a main restrictor in the damper. The pressure regulation is brought about by an equilibrium of forces between forces caused by pressure in a damper chamber and, among other things, force of a pilot valve, which is controlled by a solenoid that is electrically coupled to an adjusting device. In SE 443622, the pilot valve is continuously adjusted by a control signal, while the adjustment in SE 446514 and GB 2378231 is performed manually. The signal delivered by the control unit can be pre-programmed to adjust the damper so that the settings can be varied. For example, the settings can be varied according to the driver's style of driving, the vehicle performance, the state of the road or the like. In SE 443622 and SE 446514, the flow is adjusted directly via the damping piston, which divides the damping cylinder into a compression chamber and a return chamber respectively.
GB 2378231 describes a damper that has a valve for adjusting the damping forces on a compression stroke and another valve for adjusting the damping forces on a return stroke. The compression and return valves are coupled via a common chamber so that, on a return stroke, the damping medium flows between the return chamber and the compression chamber. On a compression stroke, on the other hand, the damping medium flows between the compression chamber and a medium-receiving reservoir. Two different types of valves are used for compression and return and only high-speed damping is adjustable from the damper.
In addition, U.S. Pat. No. 5,586,627 describes an electrically controlled damper that has a pressure reservoir. In FIG. 18 in U.S. Pat. No. 5,586,627, a damper is shown that is pressurized on the high-pressure side of the piston and in which the pressurized damping medium is led via a valve device to the low-pressure side of the piston. Both the compression damping and the return damping are adjusted by one and the same valve device. Other figures also show a damper that has a separate compression valve and a separate return valve, in which the pressure reservoir is intended to pressurize the oil with which the damper is filled and to accommodate changes in the volume of oil due to temperature differences or piston rod displacement.
The main problem that has to be overcome with the dampers described above is how, in a simple way, to combine the functions of a positive pressure build-up, in which the damping medium flow in both stroke directions can be adjusted quite separately and independently of one another, with a simple and inexpensive continuously adjustable electric control of the entire damping force characteristic or parts thereof.
The continuously adjustable dampers hitherto known, for example SE 443622, in which the damping force characteristic is determined by a signal from a control unit, have a certain delay before the valve reaches the required position. The lag in the system is caused by, for example, the friction of the separate valve parts against one another and, as a consequence, it takes between 8 and 12 msec for the valve to adjust to the new position. This lag is particularly manifest when only one valve determines the damping force characteristic of the damper such as, for example, when the single valve is seated in the damping piston.
Yet another problem can arise during rapid continuous adjustment of the damping force characteristic based upon output signals delivered by vehicle sensors. When the sensors detect a change in the nature of the road surface, the driver's style of driving or the like, it is desired that the electrically controlled valve can be adjusted to the optimum damping force characteristic on the next stroke. With previously known dampers, such near instantaneous adjustment is complicated because a damper without a positive pressure build-up on the low-pressure side of the piston lacks the capacity to adjust the return and compression damping quite independently of one another. To reduce the likelihood of cavitation with a damper of a hitherto known type, the damping force characteristics in the opposite stroke directions must be adjusted to correlate with one another. Preparing a valve for the next stroke without taking account of the damping force characteristic of the present stroke is therefore impossible because the stroke must be completed before the damping force characteristic of the valve can be changed. Owing to the time lag of the valves, the adjustment also occurs with a time lag, which can give rise to an unwanted damping force characteristic and a risk of cavitation, which is particularly manifest in the case of rapid strokes and changes of position.
It is desirable in the case of slow strokes and changes of position to be able to adjust the damping force characteristic simply, without significantly affecting the high-speed damping.
It is also desirable to provide an economic damper design in which the same valve construction can be used on both the compression and the return valve.